Is it safe to delete a lost+found folder?

fsck will recreate the lost+found directory if it is missing.

On startup most distributions run fsck if the filesystem is detected as not being unmounted cleanly.

As fsck creates the lost+found directory if it is missing, it will create it then and place anything that it finds into that directory.

So you can remove it but not recommended (as per Marcelo comment).

So far I was under the impression that deleting lost+found was perfectly safe, as it would be recreated by fsck whenever it is needed. But after the Ubuntu 12.10 upgrade I got this mail from cron:

/etc/cron.daily/standard:

Some local file systems lack a lost+found directory. This means if the
file system is damaged and needs to be repaired, fsck will not have
anywhere to put stray files for recovery. You should consider creating
a lost+found directory with mklost+found(8).

The following lost+found directories were not available:

/home/lost+found

The man-page of mklost+found says:

mklost+found pre-allocates disk blocks to the lost+found directory
so that when e2fsck(8) is being run to recover a filesystem, it does 
not  need to allocate blocks in the filesystem to store a large number
of unlinked files.  This ensures that e2fsck will not have to allocate
data blocks in the filesystem during recovery.

I am not sure what exactly that means, but it seems to indicate that not having lost+found might cause trouble on recovery. Furthermore it indicates that lost+found different from a regular directory in that it has preallocated blocks associated with it.

You don't want to delete the lost+found directory.
It is an important system folder and will be recreated on next boot anyway. There is some pretty good explanation of why it is there and what it does here